Light absorption apparatus

ABSTRACT

A light absorption apparatus includes a substrate, a light absorption layer above the substrate on a first selected area, a silicon layer above the light absorption layer, a spacer surrounding at least part of the sidewall of the light absorption layer, an isolation layer surrounding at least part of the spacer, wherein the light absorption apparatus can achieve high bandwidth and low dark current.

PRIORITY ENTITLEMENT

This application is a continuation application of U.S. patentapplication Ser. No. 14/940,572, entitled “LIGHT ABSORPTION APPARATUS,”filed Nov. 13, 2015, which claims the benefit of U.S. Provisional PatentApplication No. 62/078,986, filed Nov. 13, 2014; and U.S. ProvisionalPatent Application No. 62/081,574, filed Nov. 19, 2014; and U.S.Provisional Patent Application No. 62/121,448, filed Feb. 26, 2015; andU.S. Provisional Patent Application No. 62/126,698, filed Mar. 1, 2015;and U.S. Provisional Patent Application No. 62/197,098, filed Jul. 26,2015; all of which are incorporated by reference herein in theirentireties.

FIELD OF THE INVENTION

The present disclosure relates to a light absorption apparatus,especially to a semiconductor based photodiode.

DESCRIPTION OF PRIOR ART

A semiconductor based photodiode typically includes an intrinsicsemiconductor region between a P-type semiconductor and an N-typesemiconductor doping regions. The presence of an intrinsic region is incontrast to an ordinary PN diode, and the photons can be absorbed in theintrinsic region and the generated photo-carriers can be collected fromthe P-type and N-type regions.

SUMMARY OF THE INVENTION

It is an object of the present disclosure to provide a semiconductorbased photodiode with lower dark current and high absorption. Morespecifically, the photodiode includes germanium as the photo-absorptionmaterial based on a silicon substrate.

According to one aspect of the present disclosure, a method for forminga light absorption apparatus, includes: (1) forming an isolation layerabove a substrate, (2) removing part of the isolation layer to expose aselected area, (3) forming a spacer covering at least part of thesidewall of the selected area, (4) epitaxially growing a firstabsorption layer including germanium within the selected area, (5)forming a passivation layer including Silicon above the first absorptionlayer, wherein the surface leakage current can be reduced by passivatingthe first absorption layer, and a low leakage and high sensitivity lightabsorption apparatus can be formed.

According to another aspect of the present disclosure, a method forforming a light absorption apparatus, includes: (1) forming a firstdoping region at least partially embedded in a substrate, (2) forming afirst layer above the first doping region, (3) forming a second layerincluding germanium above the first layer, (4) forming a third layercovering the second layer, (5) forming a fourth layer including oxideabove the third layer, (6) forming a fifth layer including nitride abovethe fourth layer, (7) removing the fifth layer and stopping on thefourth layer, (8) forming a sixth layer above the fourth layer, whereinthe second layer has lattice mismatch to the surface of the substrate,and the sixth layer has a predetermined thickness such that apredetermined reflectivity can be achieved when an optical signalpassing and being reflected by the sixth layer, at least part of theoptical signal is absorbed by the second layer.

According to still another aspect of the present disclosure, a lightabsorption apparatus includes: a substrate, a light absorption layerabove the substrate on a first selected area, a passivation layerincluding Silicon above the light absorption layer, a spacer surroundingat least part of the sidewall of the light absorption layer, anisolation layer surrounding at least part of the spacer, wherein thelight absorption apparatus can achieve high bandwidth and low leakagecurrent.

According to still another aspect of the present disclosure, a lightabsorption apparatus includes: a substrate, a light absorption layerformed above the substrate and includes an upper part within a firstopening and a lower part within a second opening at least partiallyoverlapping with the first opening, a passivation layer includingsilicon above the upper part of the light absorption layer, a spacersurrounding at least part of the sidewall of the upper part of the lightabsorption layer, an isolation layer surrounding at least part of thespacer and the lower part of the light absorption layer, wherein lightabsorption apparatus can achieve high bandwidth and low leakage current.

BRIEF DESCRIPTION OF DRAWINGS

One or more embodiments of the present disclosure are illustrated by wayof example and not limitation in the figures of the accompanyingdrawings, in which like references indicate similar elements. Thesedrawings are not necessarily drawn to scale.

FIG. 1 shows a PIN photodiode structure.

FIGS. 2A to 2H show implementations to form a photodiode structure.

FIGS. 3A to 3C show implementations to form a counter doping layer in aphotodiode structure.

FIGS. 4A to 4C show implementations to form a diffusion control layeror/and counter doping layer in a photodiode structure.

FIGS. 5A to 5B show implementation of the structure shown in FIG. 4A.

FIGS. 6A to 6E are the sectional views illustrating the manufacturingsteps of forming a photodiode with etch/polish stopper according toanother implementation of the present disclosure.

FIGS. 7A to 7E are the sectional views illustrating the manufacturingsteps of forming a photodiode with etch/polish stopper according tostill another implementation of the present disclosure.

FIGS. 8A to 8F are the sectional views illustrating the manufacturingsteps of forming a photodiode with etch/polish stopper according tostill another implementation of the present disclosure.

FIGS. 9A to 9D are the sectional views illustrating the manufacturingsteps of forming a photodiode with conformal selective Ge etchingprocess as isolation according to an implementation of the presentdisclosure, and FIG. 9E is a sectional view showing photodiode withdoping region instead of etching process as the isolation.

FIGS. 10A to 10L are the sectional views illustrating forming aphotodiode with sidewall passivation, or/and interfacial layer, or/andmultiple layer forming steps.

FIGS. 11A to 11K are the sectional views illustrating forming aphotodiode with sidewall passivation, or/and interfacial layer.

FIGS. 12A to 12K are the sectional views illustrating forming aphotodiode with multiple layer forming steps, or/and sidewallpassivation, or/and interfacial layer.

FIG. 13 is a sectional view showing one photodiode of the presentdisclosure integrated with a transistor.

DETAILED DESCRIPTION

FIG. 1 shows a photodiode 10 a, which comprises a silicon (Si) substrate100 a, an n-type doped region 110 a within the Si substrate 100 alocated near the upper surface of the Si substrate 100 a, an intrinsicgermanium (Ge) region 130 a arranged on the upper surface of the Sisubstrate 100 a, a p-type Ge region 132 a arranged on the upper surfaceof the intrinsic Ge region 130 a and an oxide passivation 180 asurrounding the intrinsic Ge region 130 a and the p-type Ge region 132 aas well as covering the upper surface of the Si substrate 100 a.

For the structure shown as FIG. 1, a heterogeneous interface is presentbetween the Ge region 130 a and the underlying Si substrate 100 a.Heterogeneous interface can be implemented by using heteroepitaxy, atype of epitaxy performed by growing a crystalline material of differentelemental configurations than the crystalline substrate that is grownon. Examples include but are not limited to, GaN on sapphire, GaN on Si,Ge on Si. The crystalline materials can be elemental or compoundsemiconductors.

For some applications, electrically intrinsic material property isneeded on either grown film or substrate or both for better deviceperformance. Intrinsic semiconductor is a semiconductor exhibitingelectrically neutral property. Here a region with carrier concentrationbelow 10×¹⁷ cm⁻³ is considered intrinsic. However, intrinsic material issometimes difficult to obtain at the heterogeneous interface.Electrically polarized layers often unintentionally formed near theinterface due to lattice-mismatched defect formation, inter-diffusion(or cross diffusion) between two materials (Components of one materialcan sometimes become the other material's active dopants), contaminationduring film growth, or energy band alignment induced Fermi levelpinning. For example, a p-type Ge layer is generally formed at theinterface of Ge-on-Si system.

Furthermore, if such dislocations and other types of defects formed atthe heterogeneous interfaces due to lattice mismatch, are located withinthe semiconductor depletion region, it could increase the photodiode'sdark current, namely leakage current under dark condition, due totrap-assisted carrier generation and therefore degrade the performanceas well as narrows the design window. It is observed that thetrap-assisted generation mechanism can be effectively reduced bypassivating this defective region with high doping concentration suchthat the defect trap states are filled with extrinsic dopant assistedcarriers to neutralize the carrier generation rate. To achieve thisdoping passivation technique, precise dopant control in highly-defectivearea is sometimes difficult due to the nature of defect assisted dopantdiffusion. Uncontrolled dopant diffusion may cause unwanted performanceand reliability penalties, such as device responsivity degradation andyield reduction.

In some of the implementations of fabricating a Ge on Si photodiode asshown in FIG. 1, Ge mesa patterning is required to define the opticalabsorption area (namely the intrinsic Ge region 130 a in FIG. 1) if ablanket type epitaxial growth is used. Blanket type epitaxial growth isan epitaxial growth performed on the entire substrate wafer surface.Reactive ion etching (ME) and inductively coupled plasma (ICP) etchingare common methods for patterning Ge mesas after the blanket epitaxialgrowth where desired mesa sidewall angles are achieved with carefullyengineered anisotropic etch recipes. However, anisotropic etch normallyinvolves ion bombardment on patterned structures and often leads to Gesidewall surface damage. Damaged sidewall surfaces result in defects anddangling bonds that increases the photodiode dark current. To avoid suchdevice degradation, a conformal damage-free selective Ge etch approach(selective over Si) is presented to remove the anisotropic etch induceddamaged surface layers. For example, a selective etch can be defined asetch rate differences between Ge and Si larger than a 5 to 1 ratio.

For a higher operation speed photodiode or photodetector, the thicknessof the photo-sensitive layer, namely the intrinsic Ge region 130 a shownin FIG. 1, needs to be thin enough to minimize carrier transit time butat the expense of lower photo-responsivity. To improve responsivity andstill maintain high speed, an optical reflector can be placed atop thephoto-sensitive layer. The reflector materials can include onedielectric layer (ex: oxide or nitride), multiple dielectric layers,metal (ex: Aluminum), or any combinations of materials listed above.Forming such reflectors requires strict thickness tolerance (<5%) toensure the targeted reflectivity is within a desired spectrum, which canbe relatively difficult for conventional foundries. An etch or polishstopping layer is presented in this disclosure to improve the thicknessuniformity control for the desired reflector structure. Hereinafter, thelight absorption apparatus will be exemplified as a PIN photodiode.However, this particular example is not a limitation for the scope ofthe present disclosure. For example, a NIP structure can also beimplemented by certain implementations of this disclosure. Furthermore,other light absorption material such as SiGe with various Ge contentscan be used.

FIGS. 2A to 2F are the sectional views illustrating the manufacturingsteps of the light absorption apparatus according to the firstembodiment of the present disclosure, where a counter doping layer ispresented to reduce the operation bias or/and reduce the leakage currentat the heterogeneous interface. As shown in FIG. 2A (step S100), asemiconductor substrate 100 is provided and n+ doped layer 110 is formednear the upper face of the substrate 100. The n+ doped layer 110 may beformed methods such as, but not limited to, ion implantation, gas phasediffusion and epitaxial layer growth with simultaneous in-situ dopingcombined with optional thermal treatment procedures for dopant diffusionand activation. Two high doping regions 102 for reduced resistancecontacts are formed with higher doping levels than that of the n+ dopedlayer 110. For example, the doping concentration is larger than 1×10¹⁹cm⁻³ for the n+ doped layer 110, and is larger than 1×10²⁰ cm⁻³ for thecontacts 102.

In some embodiments of the present disclosure and as shown in FIG. 2A,the semiconductor substrate 100 is a bulk semiconductor substrate. Whena bulk semiconductor substrate is employed, the bulk semiconductorsubstrate can be comprised of any semiconductor material including, butnot limited to, Si, Ge, SiGe, SiC, SiGeC, InAs, GaAs, InP or other likeIII/V compound semiconductors. Multilayers of these semiconductormaterials can also be used as part of the bulk semiconductor substrate.In one embodiment, the semiconductor substrate 100 comprises a singlecrystalline semiconductor material, such as, for example, singlecrystalline silicon. In another embodiment, a semiconductor-on-insulator(SOI) substrate (not specifically shown) is employed as thesemiconductor substrate 100. When employed, the SOI substrate includes ahandle substrate, a buried insulating layer located on an upper surfaceof the handle substrate, and a semiconductor layer located on an uppersurface of the buried insulating layer. The handle substrate and thesemiconductor layer of the SOI substrate may comprise the same, ordifferent, semiconductor material. The term “semiconductor” as usedherein in connection with the semiconductor material of the handlesubstrate and the semiconductor layer denotes any semiconductingmaterial including, for example, Si, Ge, SiGe, SiC, SiGeC, InAs, GaAs,InP or other like III/V compound semiconductors. Multilayers of thesesemiconductor materials can also be used as the semiconductor materialof the handle substrate and the semiconductor layer. In one embodiment,the handle substrate and the semiconductor layer are both comprised ofsilicon. In another embodiment, hybrid SOI substrates are employed whichhave different surface regions of different crystallographicorientations. In this example, the semiconductor substrate 100 isexemplified as silicon substrate 100.

As shown in FIG. 2B (step S102), an epitaxially-deposited lightabsorption epitaxial layer (such as a Ge epitaxial layer) 130 is formedatop the doped layer 110 and further includes a counter doping layer 132between the doped layer 110 and the Ge epitaxial layer 130. In someimplementations, the thickness of the counter doping layer 132 can rangefrom 1 nm to 150 nm, dependent on the doping profile near the interface.The dopants inside the counter doping layer 132 should be able toprovide similar free carrier concentration to compensate for thebuilt-in potentials/carriers at the interface by providing oppositecharge polarity for electrical neutralization and to reduce the built-inpotential and hence the operation bias and/or leakage current. ForGe-on-Si system, the interface is naturally p-type and therefore,dopants inside counter doping layer 132 preferably are n-type dopants,for example, As, P or their combinations. The counter doping layer 132can be formed by in-situ doping during epitaxially growing the Geepitaxial layer. In an in-situ doping process, the dopant is introducedduring the deposition of the crystalline semiconductor material.Alternatively, the counter doping layer 132 can be formed otherapproaches such as, but not limited to, ion implantation with n-typedopants. The counter doping layer may be of same material as layer 130or a different material such as SiGe with various Ge contents. In someimplementations, additional layers may be added in-between layer 132 andlayer 130 to reduce layer 132 dopings from diffusion towards layer 130.For example, this optional layer may be of SiGe material with various Gecontents. After the Ge epitaxial layer 130 and the counter doping layer132 are formed, an oxide cap 138 is formed atop the Ge epitaxial layer130 to protect the Ge surface.

As shown in FIG. 2C (step S104), after the oxide cap 138 is formed,lithography and etching processes are performed to define a Ge mesaregion 140 and the counter doping mesa region 142, to expose the uppersurface portion of the silicon substrate 100.

As shown in FIG. 2D (step S106), suitable etchant is used to laterallyrecess the Ge epitaxial mesa region 140 or/and the counter doping mesaregion 142.

As shown in FIG. 2E (step S108), a passivation layer 150 is formed atopthe resulting structure to passivate the Ge surface and p-type dopedregion 144 is formed near the upper surface of the Ge mesa region 140 bysemiconductor manufacturing processes such as ion implantation. In someimplementations, the passivation layer 150 can be amorphous silicon(a-Si) or poly-crystalline silicon (poly-Si). In other implementations,during the doping process of region 144, passivation layer 150 can bedoped simultaneously and used for contacts formation including salicide.In some implementations, the area of the doped region 144 can havedifferent shape from the mesa region 140 when viewing from the top. Forexample, the shape of the mesa region 140 can be rectangular and theshape of the doped region 144 can be circular. In some implementations,the area of the doped region 144 can have similar shape as the mesaregion 140 when viewing from the top. For example, the shape of the mesaregion 140 and the doped region 144 can both be rectangular or circular.

As shown in FIG. 2F (step S110), inter-layer dielectric (ILD) layer 152is formed atop the resulting structure with a topography due to the Gemesa, and a CMP (Chemical Mechanical Polishing) process is performed toreduce the surface topography. Inter-layer dielectric (ILD) may bedeposited several times to attain the desired thickness. Afterwards,contact opens 154 are defined through lithography and etching processesto expose the highly doped silicon surface 102 and some part ofpassivation layer 150.

As shown in FIG. 2G (step S112), salicide 158 is formed atop the siliconsubstrate 102 surface by introducing metals such as Ni, Co, Ti, Ptfollowed by thermal forming processing and then removing the un-reactedparts. Then tungsten plugs (W plugs) 156 is formed within the contactopens 154 atop the silicide 158.

As shown in FIG. 2H (step S114), a metal interconnect (M1 layer) 160 isformed to provide electrical connection to external circuits. In someimplementations, if the optical signal is incident from the top of FIG.2H, an ARC coating can be added on top of the photodiode by firstetching an opening from ILD 152 on top of the Ge mesa region 140. Insome implementations, if the optical signal is incident from the bottomof FIG. 2H, an ARC coating can be added on the bottom top of thephotodiode by first thinning the substrate.

In this implementation, the counter doping mesa region 142 below the Gemesa region 140 with n-type dopants and suitable thickness (1 nm to 150nm) is formed to compensate the p-type heterogeneous interface reducethe built-in potential and hence the operation bias and/or leakagecurrent. Note that in FIGS. 2A-2H, two contact points shown for both thesubstrate contact and the upper absorption region contacts are forillustrative purpose in the 2D cross-section views. In someimplementations, single continuous contact via or ring to the substrateand the absorption region can also be formed to extract photo-generatedcarriers from the light absorption region. Also note that in FIGS.2E-2H, the passivation layer 150 shown covering the light absorptionregion is for illustrative purpose, and this passivation layer 150 canalso extend into other regions as long as it does not prohibit thetungsten plugs (W plugs) 156 or other forms of contact vias to makingelectrical connection to the doped region 102 and 144. Furthermore, thesegmented doped layer 110 in FIGS. 2E-2H are for illustrative purposesand in some implementations, it may extend into other regions. The Pdoped layer 144 in FIGS. 2A-2H are for illustrative purposes and in someimplementations, it may extend to the sidewalls of layer 140 in otherembodiments.

FIGS. 3A to 3C show other implementations to form an intrinsic regionnear the heterogeneous interface. As shown in FIG. 3A, the step S102corresponding to FIG. 2B can be further described by step S102 a withfollowing sub-steps: a counter doping layer 122 is formed on uppersurface of the Si substrate 100, where the counter doping layer 122 canbe formed by ion implantation. Afterward, a layer 130 is formed. Asshown in FIG. 3B, the step S102 corresponding to FIG. 2B can be replacedby a step S102 b with following sub-steps: a first counter doping layer122 a is formed on upper surface of the Si substrate 100, where thefirst counter doping layer 122 a can be formed by ion implantation.Afterward an epitaxially-grown Ge layer 130 is formed atop the substrate100 and the Ge epitaxial layer 130 has a second counter doping layer 132a. The heterogeneous interface is present between the first counterdoping layer 122 a (Si-based material) and the second counter dopinglayer 132 a (Ge-based material). In some implementations, the thicknessof the first counter doping layer 122 a ranges from 1 nm to 150 nm,depending on the doping profile near the interface. Moreover, thedopants inside the first counter doping layer 122 a should be able toprovide similar free carrier concentration as those built-inpotential/carriers in Ge epitaxial layer 130 but with opposite chargepolarity for electrical neutralization. In some implementations, thethickness of the second counter doping layer 132 a ranges from 1 nm to150 nm. Moreover, the dopants inside the second counter doping layer 132a should be able to provide similar free carrier concentration as thosebuilt-in potential/carriers in Si/Ge interface but with opposite chargepolarity for electrical neutralization due to P-type interface betweenSi and Ge caused by p-type interfacial defects and heterojunction holeconfinement.

As shown in FIG. 3C, the step S102 corresponding to FIG. 2B can bereplaced by a step S102 c with following sub-steps: A Si epitaxial layer120 is formed on the Si substrate 100. The Si substrate 100 can be dopedat the interface near the epitaxial layer 120. An epitaxially-grown Gelayer 130 is formed atop the Si epitaxial layer 120 and the Ge layer 130further includes a counter doping layer 132. The thickness of thecounter doping layer 132 ranges from 1 nm to 150 nm is preferably dopedby n-type dopants, for example, As, P or their combinations tocompensate the p-type interface. The counter doping layer 132 can beformed by in-situ doping during epitaxially growing the Ge layer or byion implantation with n-type impurities. In some implementations, the Siepitaxial layer 120 can provide multiple purposes including reducing thedopant diffusions from the doped substrate, or/and to reduce thejunction capacitance due to its smaller dielectric index than Ge.

In the examples shown in FIGS. 2B to 2H, the counter doping layer or thecounter doping mesa can be broadly referred to as an interfacial layerbetween a silicon layer and another epitaxial layer including germanium.The major composition of the counter doping layer can be either siliconor germanium or their alloy. In the example shown in FIG. 3A, thecounter doping layer 122 can be broadly referred to as an interfaciallayer between the silicon substrate 100 and the Ge epitaxial layer 130.In the example shown in FIG. 3B, the first counter doping layer 122 aand the second counter doping layer 132 a can be broadly referred to asan interfacial layer between the silicon substrate 100 and the Geepitaxial layer 130 even though two layers are involved in this example.In the example shown in FIG. 3C, the counter doping layer 132 can bebroadly referred to as an interfacial layer between the siliconsubstrate 100 and the Ge epitaxial layer 130 even though the Siepitaxial layer 120 is sandwiched between the silicon substrate 100 andthe counter doping layer 132. In the present disclosure, the interfacialcounter doping layer can be a single layer or multiple layers between alayer A and a layer B and provides an intrinsic region between the layerA and the layer B. Moreover, the interfacial layer is not necessary tobe in direct contact with one of the layer A and the layer B, otherlayer can be interposed between the interfacial layer and the layer A,or between the interfacial layer and the layer B as long as asubstantially intrinsic region can be present between the layer A andthe layer B.

FIGS. 4A to 4C are the sectional views illustrating the manufacturingsteps of forming a photodiode with reduced defect assisted dopantdiffusion according to an implementation of the present disclosure. Asshown in FIG. 4A, a doping layer 200 is formed atop the substratematerial 100 by either epitaxial growth or ion implantation. Then adopant control layer 210 is formed above the doping layer 200. In someimplementations, the dopant control layer 210 includes silicongermanium, the doping layer 200 includes germanium or silicon germaniumand the dopant inside the doping layer 200 includes phosphorous (P). Anepitaxial layer 130 including Ge is formed atop the dopant control layer210 as the photo-sensitive region, and a top doped layer 135 is formedatop the epitaxial layer 130. In some implementations, the dopants inthe top doped layer 135 include boron (B).

In some implementations, the doping layer 200 can be formed by drivingdopants from substrate material 100 into initially undoped 200 layerregion. The driving process can be done after dopant control layer 210and at least part of epitaxial layer 130 is formed. The material of theepitaxial layer 130 can be, but not limited to, Si, SiGe with Ge contentfrom 1% to 100%. The material of the dopant control layer 210 can be,but not limited to SiGe with Ge content less than that from epitaxiallayer 130, carbon-doped SiGe, or carbon-doped Ge. The material of thedoping layer 200 can be, but not limited to, highly-doped Ge,highly-doped SiGe with Ge content no higher than epitaxial layer 130 andno less than dopant control layer 200.

The doped layer 200 has the same electrical polarity as that of the Sisubstrate 100 (for example n-type doping). If Si substrate 100 is indirect contact with the Ge epitaxial layer 130 with lattice mismatch, itwill induce defects and lead to higher dark current and faster dopantdiffusion. As a result, the dopant control layer 210 is designed toplace near the Si/Ge interface as dopant block to allow dopants fromsubstrate 100 to be driven into the doping layer 200 only for reducingdark current generation by passivating defect stats without going deeperinto the epitaxial 130 region and leading to device degradation. In someimplementations, the doped layer 200 can function as a counter dopinglayer as described before to reduce the operation bias and/or leakagecurrent.

As shown in FIG. 4B, it is similar to FIG. 4A except the top doped layer135 is replaced by a heterogeneous top doped layer 136 with differentmaterial composition than the photo-sensitive material below. Theheterogeneous doped layer 136 is made out of Si or SiGe such that minoror zero lattice mismatch is introduced between the Ge epitaxial layer130 and the heterogeneous top doped layer 136.

As shown in FIG. 4C, it is similar to FIG. 4B except that another set ofdoping layer 200 b and dopant control layer 210 b is introduced betweenthe photo-sensitive material and the top doped layer 136 to improve theheterogeneous interface quality. The dopant control layer 210 b isintroduced to reduce the dopant diffusion from the top doped layer 136into the photo-sensitive region 130. The top doped layer 136 can includeSi, Ge or their combination. The dopants in the top doped layer 136 caninclude B, P, As and their combination. In some implementations, thedoping layer 200 b can have the same doping polarity as the top dopedlayer 136. In some implementations, the doping layer 200 b can functionas a counter doping layer as described before to reduce the interfacebuilt-in potential and reduce the operation bias.

In the examples shown in FIGS. 4A to 4B, the doped layer 200 and thedopant control layer 210 can be referred to as an interfacial layerbetween the Si substrate 100 and the epitaxial layer 130 even there aretwo layers between the Si substrate 100 and the epitaxial layer 130 inthose examples. Similarly, in the example shown in FIG. 4C, the dopinglayer 200 b and the top dopant control layer 210 b can be referred to asan interfacial layer between the top doped layer 136 and the epitaxiallayer 130. In the present disclosure, the interfacial layer can be asingle layer or multiple layers between a layer A and a layer B andcontrol dopants diffusion between the layer A and the layer B. Moreover,the interfacial layer is not necessary to be in direct contact with oneof the layer A and the layer B, other layer can be interposed betweenthe interfacial layer and the layer A, or between the interfacial layerand the layer B as long as dopant diffusion can be controlled within theinterfacial layer between the layer A and the layer B. In someimplementations, the relative position of the dopant control layer andthe doping layer can be interchanged, namely the dopant control layercan be either above or below the doping layer. In some implementations,the doping layer can function as the counter doping layer as describedbefore.

FIGS. 5A to 5B show one implementation of the structure shown in FIG.4A. An epitaxial layer 130 including intrinsic-Ge is grown on a N-typephosphorus-doped Si substrate 100 by first growing a seeding layer 200has similar material composition as the epitaxial layer 130, and thengrowing a dopant control layer 210 including Si or SiGe function toreduce phosphorus diffusion into the epitaxial Ge layer where itdiffuses fast and can compromise the desired intrinsic property. Thethickness and location of the seeding layer 200 and the dopant controllayer 210 can be well controlled during the growth. In someimplementations, the dopant control layer 210 ranges from 50 nm to 150nm and including SiGe. As shown in FIG. 5B, a top doped layer 135, whichhas opposite electrical polarity to the top layer of the Si substrate100, is formed to yield a p-i-n photodiode/photodetector structure.

FIGS. 6A to 6E are the sectional views illustrating the manufacturingsteps of forming a photodiode with etch/polish stopper according toanother implementation of the present disclosure. As shown in FIG. 6A,the process can succeed the step S106 shown in FIG. 2D. The doped region102 and 110 are omitted here for simple illustrative purpose. A firstinterfacial layer 112 is formed atop the Si substrate 100, and a secondlayer 140 including Ge is formed atop the first interfacial layer 112.The first interfacial layer 112 (shown as an dashed box) can be used forcounter doping as described with reference to FIGS. 2A to 3C, or fordiffusion control as described with reference to FIGS. 4A to 4C, or amaterial with larger dielectric index than that of the second layer 140for bandwidth adjustment. Also shown in FIG. 6A, a passivation layer 30with material such as, but not limited to, Si (amorphous orpoly-crystalline), silicon oxide, nitride, high-k dielectric, or theircombinations is formed to passivate and protect the second layer 140.

As shown in FIG. 6B, a stopping layer 32 with material such as, but notlimited to, nitride is formed as a blanket layer atop the passivationlayer 30. In some implementations, the stopping layer can also includemultiple layers including oxide and nitride. In some implementations,the thickness of the stopping layer 32 ranges typically from, but notlimited to 10 A to 2000 A, with a thickness from 100 A to 500 A beingmore typical. Afterward, an interlayer dielectric (ILD) layer 34 is thendeposited to cover the whole mesa structure and can be optionally firstpre-planarized by either reflow or chemical mechanical polish (CMP)process as shown in FIG. 6C. The ILD layer 34 uses material such as, butnot limited to, silicon oxide which has different material compositionthan the stopping layer 32. As shown in FIG. 6D, the ILD layer 34 isprocessed by CMP process until the portion of the ILD layer 34 atop thestopping layer 32 is substantially removed. If a pre-planarized processin FIG. 6C is not performed, then a single planarizing process such asCMP can be used to form the structure in FIG. 6D. More particularly, theremoval process is designed to fully terminate on top of the mesastopping layer 32 with minimum thickness loss. Namely, the removalprocess for the ILD layer 34 needs to be highly selective to thestopping layer 32. For example, the selectivity could be larger than1:5. Afterward, as shown in FIG. 6E a reflector 36 is then uniformlydeposited on top of the stopping layer 32. With this approach, thethickness uniformity of the top reflector 36 can be well controlled byfilm deposition step instead of a polish process, meaning betteruniformity control than conventional planarization process. Thereflector 36 is used for either reflection or tuning optical cavity pathlength or combinations of both. In some implementations, a reflectorincluding a metal layer on top of a dielectric layer can achieve >95%reflectivity, wherein an optical signal incident from the bottom of FIG.6E can be reflected for further absorption of the second layer 140. Insome implementations, a reflector including oxide or nitride can beformed to achieve less than 50% reflectivity and an optical signal canincident from the top of FIG. 6E. In some implementations, ananti-reflection-coating (ARC) layer can be added between the externaloptical source and the second layer 140. The reflector can include onedielectric layer (ex: oxide or nitride), multiple dielectric layers,metal (ex: Aluminum), or any combinations of materials listed above. Insome implementations, the reflector 36 can include dielectric such asoxide, or a metal layer such as Aluminum, or a metal layer on top of adielectric layer with its thickness close toquarter-effective-wavelength of the incident light. Reflectors generallyhave unique and strict thickness tolerance (<5%) to ensure high opticalyield, and in this implementation, a stopping (etch or polish stopping)layer 32 is provided in a photodiode/photodetector structure which willimprove the thickness uniformity control of the reflector structure.Note that the process flow provided above is not stated in specificorder and can be rearranged in any order. For example, another etchingprocess to further remove the stopping layer 32 can be added beforedepositing the reflector such that the CMP process induced thicknessvariation on the stopping layer 32 can be further reduced. In someimplementations, the stopping layer 32 is nitride and a wet etch processincluding phosphorus acid is used to remove the nitride beforedepositing the reflector 36. Moreover, in the above example, the secondlayer 140 forming a Ge epitaxial mesa region has lattice mismatch to thesurface of the Si substrate 100, and the reflector 36 has apredetermined thickness such that a predetermined reflectivity can beachieved when an optical signal passing and being reflected by thereflector 36, at least part of the optical signal is absorbed by thesecond layer 140.

FIGS. 7A to 7E are the sectional views illustrating the manufacturingsteps of forming a photodiode/photodetector with etch/polish stopperaccording to still another implementation of the present disclosure. Asshown in FIG. 7A, the process can succeed the step S102 shown in FIG.2B. The interfacial layer 112 can be used for counter doping asdescribed with reference to FIGS. 2A to 3C, or for diffusion control asdescribed with reference to FIGS. 4A to 4C, or be formed with largerdielectric index than that of the second layer 140 for bandwidthadjustment. A passivation layer 31 with material such as, but notlimited to, Si (amorphous or poly-crystalline) or silicon oxide ornitride or their combinations is formed on the resulting structure.Afterward, a stopping layer 33 with material such as, but not limitedto, nitride is formed atop the passivation layer. The thickness of thestopping layer 33 is typically from 10 A to 2000 A, with a thicknessfrom 100 A to 500 A being more typical. The passivation layer 31, thestopping layer 33 and the underlying Ge epitaxial layer 140 are thenpatterned simultaneously to form a mesa structure shown in FIG. 7A. Thenin FIG. 7B, a passivation spacer 35 is formed on the mesa (including thepassivation layer 31, the stopping layer 33, and the Ge epitaxial mesaregion 140) sidewall. In some implementations, the passivation spacer isformed by first conformally depositing a passivation film on the mesa toresult in a relatively thicker region near the sidewall. Then adirectional (anisotropic) etch is applied to remove the spacer materialon the field, leaving only the sidewall region with relatively thickerlayer remained as the spacer. Afterward, an interlayer dielectric (ILD)layer 34 is then deposited to cover up the whole mesa structure and canbe optionally pre-planarized by either reflow or chemical mechanicalpolish (CMP) process as shown in FIG. 7C. As shown in FIG. 7D, the ILDlayer 34 is processed by CMP process until the portion of the ILD layer34 atop the stopping layer region 33 is substantially removed, whether apre-planarized process is processed in FIG. 7C or not. If apre-planarized process in FIG. 7C is not performed, then a single polishprocess can be used to form the structure in FIG. 7D. Afterward, asshown in FIG. 7E a reflector 36 is then uniformly deposited on top ofthe stopping layer region 33, similar to what is described in FIG. 6E.

FIGS. 8A to 8F are the sectional views illustrating the manufacturingsteps of forming a photodiode/photodetector with etch/polish stopperaccording to still another implementation of the present disclosure. Theinitial steps illustrated by FIG. 8A to 8C are similar to thedescription of FIG. 7A to 7C, and in FIG. 8D, the ILD layer 34 isfurther processed by CMP process or etch back process and the processstops near the stopping layer region 33 with some over-polishing orover-etching on either the ILD 34 or the stopping layer 33. Compared toFIG. 7, in this implementation, the stopping layer 33 can be referred toas dummy stopping layer because it is removed at a later process, andtherefore, the removal process does not need to be selective to thestopping layer as described before, hence further improving the processflexibility. As shown in FIG. 8E, the dummy stopping layer 33 is thenremoved by wet chemical process or combination of wet and dry etchingprocess. The selected chemical can react only with stopping layer 33 andbe highly selective to the rest of the materials exposed such as the ILD34. For example, if the stopping layer is nitride, a phosphoric acidbased wet etch process can be used. Afterward, as shown in FIG. 8F areflector 36 is then uniformly deposited on top of the resultingstructure as described in FIGS. 6E and 7E. With this approach, thethickness uniformity of the top reflector 36 can be well controlled byfilm deposition step instead of a polish process, meaning betteruniformity control than conventional planarization process.

FIGS. 9A to 9D are the sectional views illustrating the manufacturingsteps of forming a photodiode/photodetector with conformal selective Geetch according to another implementation of the present disclosure. Asshown in FIG. 9A, the process can succeed the step S102 shown in FIG. 2Bwith an interfacial layer 40 placed between the epitaxial layer 130 andthe underlying Si substrate 100. In some implementations, the epitaxiallayer 130 includes Ge, the interfacial layer 40 can be an intrinsic Silayer functioning as a counter doping layer, a dopant diffusion layer orboth, and a passivation layer 42 made out of dielectric material isformed on the epitaxial layer 130. Afterward, as shown in FIG. 9B, thepassivation layer 42, the underlying epitaxial layer 130 and theinterfacial layer 40 are patterned fully or partially by RIE to form amesa structure 140. With the involvement of directional ion etching (ex:RIE), it causes damage zone 43 for the epitaxial layer near the mesasidewall as shown in FIG. 9B. As shown in FIG. 9C, selective Ge etch isperformed to remove the damaged zone 43. This process will cause alateral recess 45 as shown in FIG. 9C. Since the top surface of theepitaxial layer 130 is covered by dielectric passivation layer 42, theetching process is mostly active on mesa sidewall. The methods ofperforming this conformal selective Ge etch will be further discussed inthe following description. Finally, as shown in FIG. 9D, in order toform a p-i-n or n-i-p structure, a thin layer 46 of the epitaxial layernear the top surface is converted into a highly-doped layer with itselectrical polarity opposite to substrate doped region 110. Note thatthe process stated above is not limited to specific order.

With reference again to FIG. 9C, three possible methods to achieveconformal selective Ge etch can be further described. The 1st approachis using wet chemical etching to achieve conformal (isotropic) Ge etchwith selectivity to Si. A typical Ge etch is normally performed in twosteps. The 1st step is an oxidation reaction in which the etchedmaterial is converted into a higher oxidation state. The 2nd step leadsto the dissolution of oxidation products. In one implementation, the wetetch chemistry includes but not limited to NH₄OH (dissolution) and H₂O₂(oxidant). Etch rate can be controlled by the level of H₂O dilution.Moreover, this etch chemistry has etch selectivity on Si. Mixed NH4OHand H2O2 are used in Si industry for wafer cleaning and are known forvery low Si etch rate. The 2nd approach is using a fluorine, chlorine,and bromine-based ME process with downstream plasma configuration. It isobserved that Ge is more reactive on the above chemistries without theassistance of ion bombardment. The downstream plasma configuration canprovide a nearly damage free conformal etch without causing furthersidewall damages from directional ion bombardment. By properly tuningthe RIE conditions, an etch rate difference between Ge and Si of morethan 40 to 1 can be achieved using this approach. For the 3rd approach,a high temperature gas phase HCl etch is performed under reduced or lowpressure vacuum systems. HCl has gas chemistry that is capable ofetching Si and Ge. Since this is a gas phase etching without assistancefrom any directional ion bombardment, the reaction is conformal.Moreover, the activation temperatures of etching Ge and Si are verydifferent (more than 100 C), and hence when operating the etchingprocess at near 600 C, only Ge will be etched at this temperature range,thus creating an etch selectivity between Si and Ge. Moreover, the abovementioned processes can be applied to the other photodiodeimplementations mentioned elsewhere in this disclosure, for example, thefigures shown in FIGS. 6-8. For example, in FIGS. 7A and 8A, after mesaforming, the conformal selective Ge etch process can be introduced toremove part of the damaged sidewall of the second layer 140, leavingonly the undamaged second layer within a selected area, and then forminga spacer layer 35 as mentioned before covering at least part of theexposed sidewall of the second layer 140.

FIG. 9E is a sectional view showing photodiode with doping isolation. Inthis implementation, two absorption elements are defined by creating anopposite doping region between the two adjacent parts of the absorptionelements, and each element has its own top doping region and itssubstrate doping region. For example, if the substrate doping is N-typefor both elements, then the doping separation region is P-type. In someimplementations, if the light absorption region is slightly P-type, thenthe doping separation region is N-type. In some implementations, if theabsorption region includes Ge and the substrate is Si, their interfaciallayer could be P-type due to the surface trap states near the SiGeinterface, then the doping separation is N-type. The interfacial layer40 here may be introduced intentionally as dopant diffusion controllayer, or counter doping layer as described before; or it can indicatean inter-diffusion region between the upper Ge layer and Si substrateduring the Ge epitaxial growth thermal process.

FIGS. 10A to 10F are the sectional views illustrating the manufacturingsteps of forming a photodiode with sidewall passivation according to animplementation of the present disclosure. As shown in FIG. 10A, theprocess can succeed the step S100 shown in FIG. 2A, namely, a Sisubstrate 100 with doped layer 110. An isolation layer (such as a fielddielectric layer) 50 is then deposited on the upper surface of the Sisubstrate 100. As shown FIG. 10B, a selective area opening 50 a isdefined in the field dielectric layer 50 by photolithography andetching, where the selective area opening 50 a exposed a part (a firstselected area) of the surface of the doped layer 110. Afterward, asshown in FIG. 10C, a passivation layer 52 is deposited on the upper faceof the field dielectric layer 50 with the selective area opening 50 a.In some implementations, the passivation layer 52 is Si (amorphous orpoly-crystalline), nitride, or high-k dielectric. As shown in FIG. 10D,directional etch is performed to remove part of the passivation layer 52and only passivation spacer 52 a remains on the sidewall of theselective area opening 50 a (namely the inner surface of the fielddielectric layer 50). As shown in FIG. 10E, the first light absorptionlayer (such as photosensitive material layer including Ge) isselectively grown and fills the selective area opening 50 a, and then itis planarized by CMP process to form a photo-sensitive region 54. Beforeselectively growing the first light absorption layer, an interfaciallayer 112 may be optionally formed atop the first selected area. Theinterfacial layer 112 can be a counter doping layer as described withreference to FIGS. 2A to 3C, or dopant diffusion control layer asdescribed with reference to FIGS. 4A to 4C, or a bandwidth adjustmentlayer with smaller dielectric index than that of the photo-sensitiveregion 54. Finally, as shown in FIG. 10F, a doped region 56 is formedwithin the photo-sensitive region 54 near the surface. In someimplementations, the area of the doped region 56 can have differentshape from the photo-sensitive region 54 when viewing from the top. Forexample, the shape of the photo-sensitive region 54 may be rectangularand the shape of the doped region 56 may be circular. In someimplementations, the selective area opening 50 a is rectangular andsurrounded by (110) planes wherein when filled by a photo-sensitiveregion includes Ge can result in good surface passivation. In certainembodiments, other planes other than (110) can also be used to form therectangular. To reduce the junction capacitance, the doped layer 56 issmaller than the rectangular opening 50 b, and may be in a circularshape to substantially match the input optical beam profile. In someimplementations, the area of the doped region 56 can have similar shapeas the photo-sensitive region 54 when viewing from the top. For example,the shape of the photo-sensitive region 54 and the doped region 56 canboth be rectangular or circular. In some implementations, the shape ofthe photo-sensitive region 54 b and the doped layer 56 can bothrectangular with rounded corners.

Alternatively, as shown in FIG. 10G, the process can be bifurcated fromthe step after FIG. 10B, a seeding layer 58 is first grown within theselective area opening 50 a (seeding area). A CMP process may beperformed after the growth of the seeding layer. Subsequently, a secondisolation layer 501 is deposited on the resulting structure partiallyremoved to expose a second selected area. As shown in FIG. 10H, a spacer520 is formed on the sidewall corresponding to the second selected area.The exposed second selected area is then filled by the photo-sensitiveregion 54 and a passivation layer is deposited atop the photo-sensitiveregion 54, as shown in FIG. 10I. The steps described in FIGS. 10H and10I are similar to the steps described in FIG. 10B to 10F except aseeding layer is first grown within a seeding area before performing thesteps to form the spacer.

Alternatively, as shown in FIG. 10J, the process can be bifurcated frombefore step FIG. 10G. Prior to seeding layer filling of the selectivearea opening 50 a (seeding area), a bottom spacer 52 a can be formed onthe sidewall. As shown in FIG. 10K, a seeding layer 58 is grown withinthe selective area opening 50 a (seeding area) and a CMP process can beperformed after the growth of the seeding layer. At this point, thesteps described in FIG. 10H to 10I may be performed to form the upperlayer of photo-sensitive region 54. Subsequently, processes similar tothose described in FIGS. 2F to 2H or other variations can be applied toform the electrical contacts of the photodiode. In some implementations,the seeding layer may be Si, Ge, or SiGe with various Ge content. Thephoto-sensitive region may be Si, Ge or SiGe with various Ge content. Insome implementations, the photo-sensitive region exhibits higher Gecontent than the seeding layer. Furthermore, an interfacial layer may beinserted between the seeding layer and the substrate, or/and between thephoto-sensitive region and the substrate, or/and between the seedinglayer and the photo-sensitive region. In some implementations, theinterfacial layer can function as a counter doping layer, or/and adopant diffusion layer, or/and a bandwidth adjustment layer. In someimplementations, the seeding layer is of Si material functioning as adopant diffusion control layer to reduce the dopant diffusion from thesubstrate into the photo-sensitive region. In some implementations, theseeding layer has substantially the same material content as thephoto-sensitive region such as Ge. In some implementations, the seedinglayer may be grown separately from the photo-sensitive region above theseeding layer to reduce the thermal budget since other process stepssuch as forming silicide may be performed between the two growths. Suchtwo-step growth method enables higher overall achievable thickness forthe photo-sensitive region of substantially the same materialcompositions. In some implementations, an intentional or unintentionalsidewall misalignment exists between the seeding area and the secondselective area due the involvement of multiple lithography steps. Notethat the drawings shown in FIG. 10 are for illustrative purpose andshould not be viewed in a restrictive sense. For example, in FIGS. 10Hand 10I, the spacer formation may also be optional in this two-stepdeposition/growth scenario, namely first forming a seeding region 58,then forming a second photo-sensitive region 54 without introducing thespacer 520. As another example, the thickness of the photo-sensitiveregion 54 may be thicker than the thickness of the seeding region 58,and the opening area for the photo-sensitive region 54 may be larger,equal, or smaller than the seeding region 58.

FIGS. 11A to 11G are the sectional views illustrating the manufacturingsteps of forming a photodiode with sidewall passivation according toanother implementation of the present disclosure. Similarly, the processcan succeed the step S100 shown in FIG. 2A, namely, a Si substrate 100with doped layer 110 and a field dielectric layer 50 deposited on thesurface of the Si substrate 100, as shown in FIG. 11A. In FIG. 11B, aselective area opening 50 a is defined in the field dielectric layer 50by photolithography and etching, where the selective area opening 50 aexposed a part of the surface of the Si substrate 100.

Subsequently, as shown in FIG. 11C, photosensitive materials such as Geor SiGe of various Ge content is selectively grown to at least partiallyfill the selective area opening 50 a to form a first photo-sensitiveregion 54 a, which functions as a seed layer and will be described inmore detail. In other implementations, prior to selectively growing thefirst photo-sensitive region 54 a, an interfacial layer 112 may beformed as a counter doping layer as described with reference to FIGS. 2Ato 3C, or as a diffusion control layer as described with reference toFIGS. 4A to 4C.

As shown in FIG. 11D, a passivation layer 53 is deposited on the uppersurface of the field dielectric layer 50 and the upper surface of thefirst photo-sensitive region 54 a. The material of the passivation layer53 may include Si (amorphous or poly-crystalline), oxide, nitride,high-k dielectric or their combinations. As shown in FIG. 11E,directional etch is performed to remove part of the passivation layer 53and only passivation spacer 53 a remains on the sidewall of theselective area opening 50 a. As shown in FIG. 11F, photosensitivematerial such as Ge is selectively grown to fill the remaining part ofthe selective area opening 50 a to form a second photo-sensitive region54 b, and the second photo-sensitive region 54 b is then planarized byCMP process. Finally, as shown in FIG. 11G, a doped layer 56 is formedwithin the second photo-sensitive region 54 b and near the upper surfaceof the second photo-sensitive region 54 b. In some implementations, thedoping type of doped layer 56 is p-type, and the doping type of thedoped region 110 is n-type. In some implementations, the doping type ofdoped layer 56 is n-type, and the doping type of the doped region 110 isp-type. In some implementations, the area of the doped layer 56 can havedifferent shape from the photo-sensitive region 54 b when viewing fromthe top. For example, the shape of the photo-sensitive region 54 b maybe rectangular and the shape of the doped layer 56 may be circular. Insome implementations, the selective area opening 50 a is rectangular andsurrounded by (110) planes wherein when filled by a photo-sensitiveregion includes Ge can result in good surface passivation. In certainembodiments, other planes other than (110) can also be used to form therectangular. To reduce the junction capacitance, the doped layer 56 issmaller than the rectangular opening 50 b, and may be in a circularshape to substantially match the input optical beam profile. In someimplementations, the area of the doped layer 56 can have similar shapeas the photo-sensitive region 54 b when viewing from the top. Forexample, the shape of the photo-sensitive region 54 b and the dopedlayer 56 can both be rectangular or circular. In some implementations,the shape of the photo-sensitive region 54 b and the doped layer 56 canboth rectangular with rounded corners. Upon completion of process stepsshown in FIG. 11G, subsequent processes similar to those shown in FIGS.2F to 2H or other variations may be performed to form the electricalcontacts of the photodiode.

FIG. 11H is a section view showing the photodiode with sidewallpassivation according to still another implementation of the presentdisclosure. The photodiode shown in FIG. 11H is similar to that shown inFIG. 11G except a passivation layer 150 is formed atop the doped layer56 and the second photo-sensitive region 54 b. FIG. 11I is a sectionview showing the photodiode with sidewall passivation according to stillanother implementation of the present disclosure. The photodiode shownin FIG. 11I is similar to that shown in FIG. 11G except that theinterfacial layer 112 can be omitted in this implementation. FIG. 11J isa section view showing the photodiode with sidewall passivationaccording to still another implementation of the present disclosure. Thephotodiode shown in FIG. 11J is similar to that shown in FIG. 11G exceptthat the interfacial layer 112 is placed between the firstphoto-sensitive region 54 a and the second photo-sensitive region 54 b.FIG. 11K is a section view showing the photodiode with sidewallpassivation according to still another implementation of the presentdisclosure. The photodiode shown in FIG. 11K is similar to that shown inFIG. 11G except that two interfacial layers 112 a and 112 b are employedin this implementation. Namely, the first interfacial layers 112 a isplaced between the first photo-sensitive region 54 a and the secondphoto-sensitive region 54 b, and the second first interfacial layers 112b is placed between the doped layer 110 and the second photo-sensitiveregion 54 b. Note that in FIG. 11, the illustration of a complete layerregion and a dashed box region both indicates the existence of aninterfacial layer 112. The aforementioned “surface of the Si substrate100” is interchangeable with “surface of the doped layer 110” in certainimplementations.

FIGS. 12A to 12G are the sectional views illustrating the manufacturingsteps of forming a photodiode with sidewall passivation according toanother implementation of the present disclosure. Similarly, the processcan succeed the step S100 shown in FIG. 2A. In FIG. 12A, a Si substrate100 with doped layer 110 and a field dielectric layer 51 a deposited onthe surface of the Si substrate 100. A selective area opening 50 a isdefined in the field dielectric layer 51 a by photolithography andetching, where the selective area opening 50 a exposed a part of thesurface of the Si substrate 100. Subsequently, a seeding layer 54 a isselectively grown to fill the selective area opening 50 a. In someimplementations, before selectively growing the first photo-sensitiveregion 54 a, an interfacial layer 112 can be formed as a counter dopingas described with reference to FIGS. 2A to 3C, or as a diffusion controlas described with reference to FIGS. 4A to 4C. In some implementations,an optional CMP process can be performed after the growth of the seedinglayer 54 a. In FIG. 12B, a second isolation layer 51 b is deposited, andpart of the second isolation layer 51 b is removed to expose a secondselected area 50 b as shown in FIG. 12C. Note that in certain actualprocess implementations, a sidewall misalignment may exist between theselective opening 50 a and the second selected area 50 b due to twoseparate lithography steps involved.

In FIG. 12D, a passivation layer 53 is deposited. In someimplementations, the material of the passivation layer 53 can be Si(amorphous or poly-crystalline), oxide, nitride, high-k dielectric (ex:Al₂O₃, HfO₂) or their combinations. As shown in FIG. 12E, directionaletch is performed to partially remove the passivation layer 53 and onlypassivation spacer 53 a remains on the sidewall of the selective areaopening 50 b (namely the inner surface of the field dielectric layer 51b).

As shown in FIG. 12F, photosensitive material such as Ge is selectivelygrown to fill the remaining part of the selective area opening 50 b toform a second photo-sensitive region 54 b, and the secondphoto-sensitive region 54 b is then planarized by CMP process. Finally,as shown in FIG. 12G, a doped layer 56 is formed within the secondphoto-sensitive region 54 b and near the upper surface of the secondphoto-sensitive region 54 b. In some implementations, the doping type ofdoped layer 56 is P-type, and the doping type of the doped region 110 isn-type. In some implementations, the doping type of doped layer 56 isn-type, and the doping type of the doped region 110 is p-type. In someimplementations, the area of the doped layer 56 can have different shapefrom the photo-sensitive region 54 b when viewing from the top. Forexample, the shape of the photo-sensitive region 54 b may be rectangularand the shape of the doped layer 56 may be circular. In someimplementations, the selective area opening 50 b is rectangular andsurrounded by (110) planes wherein when filled by a photo-sensitiveregion includes Ge can result in good surface passivation. In certainembodiments, other planes other than (110) can also be used to form therectangular. To reduce the junction capacitance, the doped layer 56 issmaller than the rectangular opening 50 b, and may be in a circularshape to substantially match the input optical beam profile. In someimplementations, the area of the doped layer 56 may have similar shapeas the photo-sensitive region 54 when viewing from the top. For example,the shape of the photo-sensitive region 54 and the doped layer 56 canboth be rectangular or circular. For example, the shape of thephoto-sensitive region 54 and the doped layer 56 can both be rectangularwith rounded corners. Upon completion of process steps shown in FIG.12G, subsequent processes similar to that shown in FIGS. 2F to 2H orother variations may be performed to form the electrical contacts of thephotodiode.

FIG. 12H is a section view showing the photodiode with sidewallpassivation according to still another implementation of the presentdisclosure. The photodiode shown in FIG. 12H is similar to that shown inFIG. 12G except a passivation layer 150 is formed atop the doped layer56 and the second photo-sensitive region 54 b. FIG. 12I is a sectionview showing the photodiode with sidewall passivation according to stillanother implementation of the present disclosure. The photodiode shownin FIG. 12I is similar to that shown in FIG. 12G except that theinterfacial layer 112 can be omitted in this implementation. FIG. 12J isa section view showing the photodiode with sidewall passivationaccording to still another implementation of the present disclosure. Thephotodiode shown in FIG. 12J is similar to that shown in FIG. 12G exceptthat the interfacial layer 112 is placed between the firstphoto-sensitive region 54 a and the second photo-sensitive region 54 b.FIG. 12K is a section view showing the photodiode with sidewallpassivation according to still another implementation of the presentdisclosure. The photodiode shown in FIG. 12K is similar to that shown inFIG. 12G except that two interfacial layers 112 a and 112 b are employedin this implementation. Namely, the first interfacial layers 112 a isplaced between the first photo-sensitive region 54 a and the secondphoto-sensitive region 54 b, and the second interfacial layers 112 b isplaced between the doped layer 110 and the second photo-sensitive region54 b. Note that in FIG. 12, the illustration of a complete layer regionand a dashed box region both indicates the existence of an interfaciallayer 112. Also note that the drawings shown in FIG. 12 are forillustrative purpose and should not be viewed in a restrictive sense.For example, in FIGS. 12D to 12K, the spacer formation may also beoptional in this two-step deposition/growth scenario, namely firstforming a seeding region 54 a, then forming a second photo-sensitiveregion 54 b without introducing the spacer 53 a. As another example, thethickness of the photo-sensitive region 54 b can be thicker or thinnerthan the thickness of the seeding region 54 a, and the opening area forthe photo-sensitive region 54 b can be larger, equal, or smaller thanthe seeding region 54 a. The aforementioned “surface of the Si substrate100” is interchangeable with “surface of the N doped layer 110” incertain embodiments.

FIG. 13 is a sectional view showing the photodiode of the presentdisclosure integrated with a transistor. A high doping region isprovided at the substrate 100 for the source 72 and drain 74 region of atransistor 70. The isolation between the photodiode and the transistorcan be done by shallow trench isolation, P-N junction isolation, thermaloxide or other forms of isolation.

Although the present invention has been described with reference tospecific exemplary embodiments, it will be recognized that any and allof the implementations described above can be combined with each other,and the invention is not limited to the implementations described, butcan be practiced with modification and alteration within the spirit andscope of this invention. Accordingly, the specification and drawings areto be regarded in an illustrative sense rather than a restrictive sense.

In some implementations, the materials of the interfacial layer, theseeding layer and the photo-sensitive region can be Si, Ge or SiGe withvarious Ge content. In some implementations, the photo-sensitive regionhas higher Ge content than the seeding layer. Furthermore, aninterfacial layer can be inserted between the seeding layer and thesubstrate, or/and between the photo-sensitive region and the substrate,or/and between the seeding layer and the photo-sensitive region. In someimplementations, the interfacial layer can function as a counter dopinglayer, or/and a dopant diffusion layer, or/and a bandwidth adjustmentlayer. In some implementations, the seeding layer includes Si andfunctions as dopant diffusion control layer to reduce the dopantdiffusion from the substrate into the photo-sensitive region.

In some implementations, the seeding layer has substantially the samematerial content as the photo-sensitive region such as Ge. In certainembodiments, the seeding layer may be grown separately from thephoto-sensitive region above the seeding layer to reduce the thermalbudget since other process steps such as forming silicide may beperformed between the two growths. Such two-step growth method enableshigher overall achievable thickness for the photo-sensitive region ofsubstantially the same material compositions. In some implementations,an intentional or unintentional sidewall misalignment exists between theseeding area and the second selective area due to the multiplelithography steps involved. In some implementations where selectivegrowth is performed, a sloped shape can be formed during the growthstep, and later be polished by CMP step as mentioned in this disclosure.For example, a (311) plane can be formed if Ge is selectivelyepitaxially grown on a surface. In some implementations, a recessedstructure can be included before forming the substrate doping and theinterfacial layer. In some implementations, a spacer can also be formedto cover the sidewall of the recess area according to the spacerformation process described before. Also note that certain actualimplementation induced imperfections should also be covered in thisdisclosure as long as its concept follows this disclosure. Anyvariations, derivations from the description above should also be viewedas included in this invention.

While this specification contains many details, these should not beconstrued as limitations, but rather as descriptions of featuresspecific to particular embodiments. Certain features that are describedin this specification in the context of separate embodiments orimplementations may also be implemented in combination in a singleembodiment. Conversely, various features that are described in thecontext of a single embodiment may also be implemented in multipleembodiments separately or in any suitable subcombination. Moreover,although features may be described above as acting in certaincombinations and even initially claimed as such, one or more featuresfrom a claimed combination may in some cases be excised from thecombination, and the claimed combination may be directed to asubcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. In certain circumstances, multitasking and parallel processingmay be advantageous. Moreover, the separation of various systemcomponents in the embodiments described above should not be understoodas requiring such separation in all embodiments.

Thus, particular embodiments have been described. Other embodiments arewithin the scope of the following claims. For example, the actionsrecited in the claims may be performed in a different order and stillachieve desirable results.

What is claimed is:
 1. A method of forming a light absorption apparatus,the method comprising: doping a surface of a substrate to form a dopedlayer in the substrate; and forming, atop the doped layer, aphotosensitive structure that includes (1) a counter doping layer at ornear the bottom of the photosensitive structure; and (2) a dopantcontrol layer atop the counter doping layer, wherein the dopant controllayer has a material that retards dopant diffusion from the counterdoping layer into an intrinsic layer in the photosensitive structure,wherein a material for the counter doping layer is one or more of:highly-doped Ge, or highly-doped SiGe with a Ge content no less than thedopant control layer, wherein the substrate is silicon based, andwherein both the counter doping layer and the intrinsic layer isgermanium based.
 2. The method of claim 1, wherein the dopant controllayer is located in between the counter doping layer and the intrinsiclayer.
 3. The method of claim 1, wherein said material that retardsdopant diffusion is one or more of: silicon (Si), silicon germanium(SiGe) with a germanium (Ge) content less than the intrinsic layer,carbon-doped SiGe, or carbon-doped Ge.
 4. The method of claim 1, whereina heterogeneous interface between the substrate and the photosensitivestructure has a lattice mismatch more than 0.2%.
 5. The method of claim1, further comprising: forming another dopant control layer atop theintrinsic layer.
 6. The method of claim 1, wherein the counter dopinglayer includes dopants to compensate for a built-in electrical potentialat a heterogeneous interface between the photosensitive structure andthe substrate.
 7. The method of claim 6, wherein the dopants in thecounter doping layer are configured to provide similar free carrierconcentration to built-in carriers in a photosensitive material in thecounter doping layer but with opposite electrical polarity.
 8. Themethod of claim 1, further comprising: forming a stopping layer abovethe photosensitive structure; forming a dielectric layer atop thestopping layer; planarizing a wafer carrying the light absorptionapparatus by removing the dielectric layer, wherein the planarizing stepis to stop at the stopping layer; and forming a reflector layer abovethe photosensitive structure.
 9. The method of claim 1, the methodfurther comprising: after mesa patterning for the photosensitivestructure, performing a selective etching process to remove at least aportion of the intrinsic layer damaged by reactive ion etching performedduring the forming of the photosensitive structure.
 10. The method ofclaim 9, wherein the selective etching process is with a selectivity ofat least 1:5, the selectivity being a ratio of polish rate of silicon togermanium.
 11. A light absorption apparatus comprising: a substratehaving a doped layer; and a photosensitive structure atop the dopedlayer, the photosensitive structure including (1) a counter doping layerat or near the bottom of the photosensitive structure; and (2) a dopantcontrol layer atop the counter doping layer, wherein the dopant controllayer has a material that retards dopant diffusion from the counterdoping layer into an intrinsic layer in the photosensitive structure,wherein a material for the counter doping layer is one or more of:highly-doped Ge, or highly-doped SiGe with a Ge content no less than thedopant control layer, wherein the substrate is silicon based, andwherein both the counter doping layer and the intrinsic layer isgermanium based.
 12. The apparatus of claim 11, wherein the dopantcontrol layer is located in between the counter doping layer and theintrinsic layer.
 13. The apparatus of claim 11, wherein said materialthat retards dopant diffusion is one or more of: silicon (Si), silicongermanium (SiGe) with a germanium (Ge) content less than the intrinsiclayer, carbon-doped SiGe, or carbon-doped Ge.
 14. The apparatus of claim11, wherein a heterogeneous interface between the substrate and thephotosensitive structure has a lattice mismatch more than 0.2%.
 15. Theapparatus of claim 11, further comprising: another dopant control layeratop the intrinsic layer.
 16. The apparatus of claim 11, wherein thecounter doping layer includes dopants to compensate for a built-inelectrical potential at a heterogeneous interface between thephotosensitive structure and the substrate.
 17. The apparatus of claim16, wherein the dopants in the counter doping layer are configured toprovide similar free carrier concentration to built-in carriers in aphotosensitive material in the counter doping layer but with oppositeelectrical polarity.
 18. The apparatus of claim 11, further comprising:a stopping layer above the photosensitive structure.